The present invention relates to a plant for transmitting electric power comprising a direct voltage network for High Voltage Direct Current (HVDC) and at least one alternating voltage network connected thereto through a station. The station has at least one VSC-converter adapted to convert direct voltage into alternating voltage and the converse, and for transmitting electric power between the direct voltage network and the alternating voltage network.
Such a plant has recently become known through the thesis "PWM and Control of Two and Three Level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, in which publication such a plant for transmitting electric power through a direct voltage network for HVDC is described. Before the issuance of the thesis, plants for transmitting electric power through a direct voltage network for HVDC have been based upon the use of line-commutated CSC (Current Source Converter) converters in stations for power transmission. By the development of IGBTs (Insulated Gate Bipolar Transistor; a bipolar transistor having an insulated gate) for high voltage applications, and the suitability to connect them in series in valves in converters, since they may easily be turned on and turned off simultaneously, VSC (Voltage Source Converter) converters for forced commutation have now become an alternative. This type of transmission of electric power between a direct voltage network for HVDC, being voltage-stiff therethrough, and alternating voltage networks connected thereto, offers several important advantages with respect to the use of line-commutated CSCs in HVDC. Thus, the consumption of active and reactive power may be controlled independently of each other, and there is no risk of commutation failures in the converter, and thereby no risk of transmission of commutation failures between different HVDC links, which may occur in line-commutation. Furthermore, there is the possibility to feed a weak alternating voltage network or a network without any generator of its own (a dead alternating voltage network). Further advantages are also provided.
In a plant of this type, it is desirable to operate without expensive transformers and still obtain the exact voltage asked for on the direct voltage network.